There are many fields in which it is desirable to use polyester coating compositions for purposes of protection and for aesthetic reasons.
For reasons of economy, it is important that such coating compositions be applied rapidly and efficiently. In particular, with the constant striving for higher productivity in industry, methods of applying coatings (e.g., paints) to yield a standard film thickness in two coats instead of three or more and which still produce a serviceable coating is clearly a desirable goal to those concerned with application of coatings in production.
In the automotive industry, for example, which is one of the important applications for polyester coatings, the problem of coating application acquires specific urgency. Competitive pressure requires the original production finishing of automobiles to make the most effective use of labor and materials. Economizing production line space and the capability of increasing speed of unit operations such as painting therefore become important. Topcoat finishing, for example, that can achieve the same protective film thickness and beauty with two spray applications when three or more are standard practice is regarded as a valuable modification.
Prior to this invention, commercial polyester enamel topcoats required at least three spray applications, particularly when pigmentation containing metallic flake, to deposit films of requisite thickness and appearance. This invention concerns the solution of the above-mentioned problems and, additionally, realizes further advantages to be mentioned below.
The invention is primarily concerned with improvement in polyester enamels such as increased efficiency of application, the ability to apply a satisfactory coating in two coats rather than three, and the achievement of high gloss and excellent metallic pattern control to give an aesthetically pleasing appearance.
It has now been found that the addition of insoluble crosslinked polymeric microparticles to solutions or dispersions of polyester resins results in compositions which are capable of being sprayed to a high film build in two coats with increased coating efficiency and the films formed have dramatically improved pattern control and resistance to solvent popping while film gloss is maintained. The ability to add such crosslinked polymeric microparticles to polyester coatings without decreasing gloss is unexpected and surprising since additives to such compositions heretofore generally exerted a dulling (i.e., flatting) effect on the film.